The penetrating nature of dilepton radiation is also reflected in the centrality dependence of the excess yield in the LMR.
While the observed hadron multiplicities are understood to scale with the volume of the fireball, for given freeze-out temperature and chemical potentials, the dilepton yield is also determined by the lifetime of the radiating fireball.
Moreover, the emission rate is strongly depending on the temperature of the emitting source which of course changes in the course of fireball evolution.
A simple evolution of the excess radiation as a function of centrality is therefore not to be expected.
Fig. REF  shows an attempt to establish the systematics of the ratio \(R\)  of signal over cocktail obtained in various experiments.
The collision energies span two orders of magnitude while mostly heavy collision system are investigated, besides the medium size In-In system explored by Na60.
Because of the wide range in collision energy, the definition of the “excess” is not strictly unique.
The "low energy" case is represented by the HADES Au-Au data taken at \(\sqrt{s_{\mathrm {NN}}}\, = 2.4 \, \mathrm {G}e\mathrm {V}\) .
Four classes of centrality are shown, \(0-10\) %, \(10-20\) %, \(20-30\) % and \(30-40\) % of the total geometrical cross section. The centrality classes are obtained from fits of Glauber MC simulations to the measured charged particle multiplicities in the HADES acceptance.
Details of the centrality selection can be found in [1]}.
For the systematics presented here, \(R\)  is taken to be the signal yield integrated in the mass range 200 M\(e\) V/\(c^{2}\)  \(\le M_{\mathrm {e}\,\mathrm {e}} \le 600\)  M\(e\) V/\(c^{2}\)  divided by the respective integral over the sum of meson cocktail and the N-N reference spectrum in the same mass range.
The yields are derived in the acceptance which, in the case of HADES, covers a large fraction of the phase space around mid-rapidity.
The ratio is plotted as a function of the multiplicity of produced charged particles.
We have chosen to do so since at this energy the final state is dominated by the stopped baryons and hence not representing a measure for the event activity.
Hence, the three data points for HADES are placed at the respective charged pion multiplicity in full phase space.
Pions are the only abundantly produced hadrons at the SIS18 energy domain.
